The use of electrical of electronic liquid crystal display devices for showing characters, and in particular, numbers is already well-known. Such devices besides being compart, rapid in operation and relatively inexpensive have the advantage that the characters displayed are rendered visible by incident light so that no special light source is needed, and furthermore, the characters are visible in a brightly lit room.
Since characters on a liquid crystal display device are rendered visible by incident light, where the ambient light level is low, it is desirable that steps be taken to enhance the visibility of such characters. One method that has been used is to make the rear plate of the display device of a reflective metal. Another similar method has been to make the segmented electrodes of a reflective metal and to place the segmented electrodes on the interior surface of the rear plate of the display device. Both of these arrangements suffer from the problem that if any trace of moisture is present in the liquid crystal material itself then electro-chemical reactions can proceed which may result in the degradation of the reflectance of the electrodes, even to the point where they are completely non-reflective. The electro-chemical process which is particularly to be feared is the electrolysis of moisture to produce oxygen which may react with the surface of the metal forming a non-reflective oxide.
Heilmeier et al. in U.S. Pat. No. 3,499,112 have disclosed a display device in which the entire rear surface of the display device is reflective. However, they recommend that a collimated beam be used for illuminating the device and that the beam strike the device at such an angle that there will be no direct reflection to the eye of the observer. The invention of Heilmeier et al is designed for the display of only a single point at any given instant. Needless to say, direct reflection from the remainder of the unactivated portion of the device would make it extremely difficult to discern the isolated point being activated.